When wireless power transmission is performed by using magnetic resonance, the resonance frequency of a coil may deviate from a designed value due to manufacturing unevenness or an external environment such as a near desk. As a result, the transmission efficiency deteriorates. In addition, if the frequency with which transmission is performed is changed, there is a need to vary the resonance frequency of a coil.
Conventionally, a method has been reported in which a capacitor is attached to a coil and a value of the capacitor is changed to vary the resonance frequency of a coil. However, this method poses a problem that parasitic resistance of a capacitor increases loss at the coil, leading to deteriorated transmission efficiency.
In addition, a method for inserting a capacitor in series with a loop coupled to a coil has been reported. However, this method poses a problem that an amount that can be adjusted is limited.
Furthermore, there are objects of improving a variable amount of resonance frequency and achieving the size reduction, the thickness reduction, the weight reduction, the lowered loss, the lowered cost, and the increased electric power of a coil.